1. Field of the Invention
The present invention relates to digital video signal processing; and more particularly, to a device for splitting a screen expressed in moving picture signals compressed by MPEG (Moving Picture Expert Group) on a completely compressed domain by employing an non-transcoding technique, in image processing functions of a Multipoint Control Unit (MCU) which is adapted to provide multipoint multimedia communication services to a plurality of multipoint multimedia communication terminals, in broadband ISDN, and the method thereof.
2. Description of the Prior Art
Recently, multimedia services in the broadband ISDN have been further embodied to provide improved serviceable functions to users, as applications such as Asynchronous Transfer Mode (ATM) switches and transmission systems corresponding to a lower concept in the broadband ISDN have been broadly studied and Audio/Video compressed standard is determined by ITU-T, ISO/IEC.
In future, various multimedia services may be extended to applications such as communication/distribution services, e.g., Video-On-Demand (which is referred to as "VOD" hereinafter) and interactive series, e.g., video-telephone and Tele-conference.
The interactive multimedia communication services has been embodied recently by the ITU-T a standard for multimedia communication terminals with H.320 series available in various networks, based on H.261 image compression coding technique. In Digital Audio Video International Council (DAVIC), a standard of VOD type of service system have been defined through the use of the MPEG standard of ISO/IEC. In a type of the multimedia communication terminals, the interactive services employs the H.261 standard compression technique and the communication/distribution service applications such as the VOD incorporates the MPEG standard compression technique. On the other hand, various communication services and multimedia communication terminals, which is combined together with standards previously described, may be introduced as a terminal based on the MPEG standard of ISO/IEC is standardized by the ITU-T, and an interactive bi-directional communication service is utilized to the DAVIC and the like.
More particularly, MPEG-2 in the ISO/IEC have being directly applied to service applications such as Cable Television (CATV), digital TV broadcast and the VOD, wherein the services is available in terms of Set-Top Box (which is referred to as "STB" hereinafter) already completed a specification and a standard. Such STM may be incorporated applications such as TV, Video Cassette Recorder (VCR) and Personal Computer (PC) which have broadspreadly been popularized to individual user. Based on the STB, users are capable of utilizing various multimedia communication services on the broadband ISDN on a real time basis. Provision of the interactive communication series such as video-telephone and Tele-conference to an user, however, is possible by using only H.320 terminals according to a type of corresponding network. As a result, there is a drawback that the users currently using such STB have to purchase the H.320 terminals to use the interactive communication services. In this case, therefore, an MPEG coding module is applied to the STB incorporating therein a network interface function and an MPEG decoder, thereby allowing the user to efficiently utilize the bi-directional multimedia communication service. In the multimedia communication terminals, various types of media signals and it combined signals may be utilized depending on service applications and characteristics of devices incorporated therein. The MCU for providing the multipoint multimedia communication service has a need of structure adapted to various media signal processing. To accomplish this, the present invention provides an MPEG based moving picture processing among various media signal processing functions.
The conventional Multipoint Control Unit incorporating an image processor therein has two functions; i.e., one is a switching function of switching moving pictures generated from a plurality of multimedia communication terminals connected thereto to distribute to a preset location, and the other is a screen splitting function of allowing a multiplicity of input moving pictures to be displayed on a simple screen.
In the screen splitting function, there is a merit that the MCU carries out the multiplicity of input moving pictures and transmits the same to each terminal, to thereby allow each terminal's user to concurrently see each of the input moving pictures on the same screen. Such screen splitting function is useful for multimedia communication service applications such as multipoint video-conference systems. Since, however, the conventional MCU incorporating the image processor therein mainly employs screen-splitting technique based on PEL (Pixel element) domain, which is provided with encoders and decoders corresponding to the number of input/output connection communication terminals, respectively, the MCU results in a completely structure.
The conventional screen-splitting technique is based on a Transcoding technique, which is converts a compressed image inputted from the MCU into an image data on the PEL domain through the use of a moving picture decoder, and performs Scaling, filtering and reordering processes on a converted PEL image data, followed by a compression process using a moving picture encoder to thereby generate a compressed signal to be transmitted to video-conference participants terminals. In such transcoding technique, however, the moving picture signal can be adaptively treated, while an image processor with a complicated structure consisting of a number of decoders and encoders must be integrated on the MCU.
In addition, an iterative performance of decoding and encoding processes results in a degradation in picture quality, i.e., degradation in a Quality of Service (QoS).
Especially, QoS of the interactive communication service on a real-time basis has a limitation, due to a delay invoked by a buffer for transmitting output generated from an encoder with a traffic characteristic of variable bit in a constant bit rate, and a delay invoked during encoding and decoding processing. In case the transcoding technique is used for the screen-splitting function of the MCU, a total delay element induced by a sequence of a transmission terminal, a network, the MCU and a reception terminal, should be considered.
In addition, the screen-splitting technique previously described has shortcomings that delay resulting from the iterative encoding and decoding operations prevents the multipoint multimedia Communication services from being provided to the users on the real time basis, wherein most of the delay is occurred between transmitting terminal and a receiving terminal except the MCU.
Further, in the conventional screen-splitting technique which is not applied the decoding and encoding operations at the completely compressed domain, there has been studied only H.261 moving picture signals, but there have not been studied with respect to MPEG based on moving pictures, which is differed from compression manner for the H.261 and also encoding syntax configuration for moving pictures.